O‐Isocyanates as Uncharged 1,3‐Dipole Equivalents in [3+2] Cycloadditions

1,3‐Dipoles are commonly used in [3+2] cycloadditions, whereas isoelectronic uncharged dipole variants remain underdeveloped. In contrast to conventional 1,3‐dipoles, uncharged dipole equivalents form zwitterionic cycloadducts, which can be exploited to build further molecular complexity. In this work, the first cycloadditions of oxygen‐substituted isocyanates (O‐isocyanates) were studied experimentally and by DFT calculations. This unique cycloaddition strategy provides access to a novel class of heterocycle aza‐oxonium ylides through intramolecular and intermolecular cycloadditions with alkenes. This allowed a systematic study of the reactivity of the transient aza‐oxonium ylide intermediate, which can undergo N−O bond cleavage followed by nitrene C−H insertion, and the formation of β‐lactams or isoxazolidinones upon varying the structure of the alkene or O‐isocyanate reagents. O‐Isocyanates undergo a novel [3+2] cycloaddition to form a zwitterionic cycloadduct: an aza‐oxonium ylide. The aza‐oxonium ylide shows divergent reactivity, thereby enabling the synthesis of a variety of saturated N‐containing heterocycles through nitrene C−H insertion, formal ring contraction, and [1,2] shift from the ylide cycloadduct.